French patent No. 989 842 describes means for reducing the amount of thermal energy consumed by the evaporation of the water contained in vegetable grains at the time of the drying required for their conservation.
This patent claims "heat exchanger elements mounted in the pre-heating area, which elements transmit to the product to be processed the heat of the vapours from the heating area".
A simple calculation shows that this device could permit an energy saving of between 10 and 20% through contact between the grains and vanes mounted on the tubes condensing a part of the steam produced by the drying of a grain containing 35% moisture.
West German patent No. 3 516 062-A1 demonstrates the same concern on the part of the inventors to save energy in the drying of granulous materials; it accomplishes the drying through contact with the walls of 186 vertical tubes and by the licking of gas, steam or air on the internal helical surface of a stream of granulates dropping between two helical walls borne by the tube. This process seems far too costly to sell easily given the present position regarding the cost of energy and appears to consume over 800 thermal KWH per ton of water evaporated.
Furthermore, the volume of gas, steam or air to be conveyed can be used only once.
Finally, as in patent No. 989 842, the recovery of the energy corresponding to the latent heat of the evaporated water is only partial and probably does not exceed 20% when operating with air or gas between 40.degree. and 90.degree. C.
A third patent, Belgian patent No. 715694 entitled "Methode et appareil de sechage en vrac par congelation" ("Method and apparatus for bulk drying by freezing") is of great interest as regards the drawings but, although this patent states, several times, that it proposes to ensure continuous circulation of the frozen granulate bodies, none of the drawings and none of the descriptions indicate the means for so doing, nor does any means indicate recycling of the drying fluid, far less any effect of fluidizing the frozen granulate bodies, by a fluid.
Finally, the apparatus described is supposed to consume between 1000 and 2000 KWH per ton of evaporated water.
The object of the present invention is to realize a far greater energy saving than that which might possibly be achieved by patent No. 989 842.
In this patent, part of the recycled air emerging saturated from the condenser tubes (4) is, in fact, superheated only once before entering the non-perforated ducts (6) to divide into two streams: one travelling upwards through all the grains and commencing the drying operation until it emerges saturated (at 5), the other travelling downwards through all the grains to complete their drying until it emerges saturated (or otherwise) in the ducts (7) and restitutes at most 0 to 30% of the energy expended during the pre-heating of the grains.
On the other hand, in the present invention, all of the drying fluid (gas, steam or air) enters saturated with steam produced by the drying at a temperature .THETA.0 defined by the thermal sensitivity of the grains to be dried, and is then superheated a great many times, without any fresh supply but taking with it the maximum quantity of energy that can be recovered by condensation. The present invention cannot be applied to sugar or other very hygroscopic products.
As an illustration of thermal "sensitivity", the temperature .THETA.0 will be set, for example, to 40.degree. C. approximately for the purpose of drying sunflower seeds, certain oleaginous seeds and seeds for sowing; to 60.degree. C. approximately to dry starch containing corn; and to 100.degree. C. or more to dry mineral or organic products that are hardly hygroscopic, such as silica sand for glassmaking or the granulates intended for coating for road surfacing purposes, or again, fodders.
However, and this is what sharply distinguishes the present invention from those of the previous two patents, the drying fluid will be slightly superheated 2N times (with N varying from 2 to 20) and saturated 2N times when passing through and drying by fluidization a thin layer of material to be dried, carried by perforated funnels and not by solid ducts as in French patent No. 989 842. To facilitate the passage of light products such as dusts, powders or follicles of corn (or of grains) upwards and to prevent as far as possible the passage of the grains to be dried downwards, the holes in the funnels will advantageously be orientated obliquely in the direction of circulation of the grains, and at the same time wide enough to let through light bodies. The same drying fluid is thus progressively laden with steam (or solvent vapour) and emerges saturated with water (or solvent) at a temperature differing as little as possible from the temperature .THETA.0. In the case of water, calculations show that, per ton of water evaporated, these products can be dried by consuming a maximum of:
140 KWH of electricity per ton of water when .THETA.0.apprxeq.40.degree. C., PA0 120 KWH of electricity per ton of water when .THETA.0.apprxeq.60.degree. C., PA0 80 KWH of electricity per ton of water when .THETA.0.apprxeq.100.degree. C.
The feasibility study shows, moreover, that, for the same equipment purchase cost, it is possible to evaporate 1 ton of water at .THETA.0=40.degree. C., 1.5 to 2 tons of water at .THETA.0=60.degree. C. and 3 to 4 tons of water when .THETA.0=100.degree. C.
Persons of ordinary skill in of the art know that the ponderal specific heat of steam is equal to 4 times the ponderal specific heat of air; they know that the thermal conductivity of water is approximately two times greater than that of air. Such persons will appreciate that the ventilation costs are almost divided by 4 upon changing from 10% water (.THETA.0=40.degree. C.) to 0% air and 100% water (.THETA.0=100.degree. C.).
Persons skilled in of the art also appreciate that the performance coefficients (COP) of the compressors range from 6 when .THETA.0=40.degree. C. to 10 or more when .THETA.0=100.degree. C.
These points explain the increasingly small specific consumption figures and ever lower installation costs as the temperature rises from 40.degree. C. to 100.degree. C.